Optical access networks are usually realized as passive optical networks (PONs). In a PON, a plurality of optical network units (ONUs) is connected to a central node, often referred to as optical line terminal (OLT), often via one or more remote nodes (RN), which comprise passive optical components only. Usually, each ONU is coupled to an RN via a single optical fiber that is used for bidirectional optical communication between the respective ONU and the OLT. For this structure of an optical access network, it is known to use an optical wavelength division multiplex (WDM) signal in the transmission path between the OLT and the respective RN or all RNs. If the RNs are connected via an optical bus or ring structure, each RN may be realized as a passive optical add/drop multiplexer adapted to extract from and add to the optical WDM signal the respective optical channel signal. In the simplest case, each optical channel signal included in the optical WDM signal may be used for establishing a bidirectional communication between an ONU and the OLT.
Of course, if the optical channel signal at a port of the RN is a time division multiplex (TDM) signal, the TDM signal may be supplied to a plurality of ONUs which are connected to the respective RN via a star structure. This structure or substructure may be realized by using a passive optical splitter or star coupler equally dividing the optical TDM (channel) signal and feeding each of the divided signals to an ONU. In this case, an asymmetrical TDM scheme must be applied as only one ONU may transmit at the same time. As each transmitter unit in an ONU must be silent at times outside of a dedicated time slot (within the asymmetrical TDM signal), it is necessary to turn the laser (or other optical transmitting element) on and off very quickly. This requires expensive active optical components (especially lasers) with extremely low transient times. In such a star structure, the simplest way of providing protection against a single point of failure, e.g. a fiber break between an ONU and the RN, is to connect the ONU to the RN via an additional fiber. However, this duplication of the transmission medium as well as the necessary parts of the RN and ONU causes a disadvantageous redundancy of resources.
European Patent Application No. EP 1746857 A1 discloses a linear bus structure for a PON comprising two OLTs, each OLT connected to the end of a linear bus or chain-like structure of ONUs. The OLTs may be present at the same location so that, in this case, the linear structure becomes an open-ring structure. In both cases, it is proposed to use specific optical coupling/decoupling means having three ports, two of which are used to connect the optical fibers between two neighboring ONUs or between an ONU and a neighboring OLT and the third for connecting the respective ONU. Here, the plurality of three-port coupling means may be considered a distributed RN. In such linear or chain-like PON structures, a symmetric TDM scheme may be used, i.e., the ONU and the OLT may use a dedicated time slot of the TDM scheme to transmit their data.
In practice, customer locations, at which ONUs are necessary, are often connected to the location of an OLT and/or to one or more other customer ONU locations simultaneously using a single optical fiber connecting the respective two locations.